1. Field of the Invention
The following description relates to a hologram optical device, a compatible optical pickup having the hologram optical device, and an optical information storage medium system employing the compatible optical pickup; and more particularly, to a hologram optical device for recording and/or reproducing information on or from a plurality of optical information storage media having different thicknesses by using light emitted from a light source, a compatible optical pickup having the hologram optical device, and an optical information storage medium system employing the compatible optical pickup.
2. Description of the Related Art
The recording capacity of an optical information storage medium, such as an optical disc, depends on the size of an optical spot of a laser beam focused by an objective lens in an optical information storage medium system that records and/or reproduces data on and/or from the optical disc using the optical spot. The size of the optical spot is determined according to the wavelength λ of the laser beam and the numerical aperture (NA) of the objective lens as shown in Equation 1:Size of Focused Optical Spot∝λ/NA  (1)
Accordingly, a short wavelength light source, such as a blue laser beam, and an objective lens having a high NA are required to reduce the size of the optical spot focused on the optical disc so as to increase the recording density of the optical disc.
According to the Blu-ray disc (BD) standard, a single side of a BD has a storage capacity of about 25 GB. The BD standard uses a light source emitting light with a wavelength of about 405 nm and an objective lens having an NA of 0.85. The thickness (corresponding to a distance from an optical incidence surface to an information storage surface, i.e., a thickness of a protective layer) of the BD is about 0.1 mm. According to the high definition digital versatile disc (HD DVD) standard, an HD DVD has the storage capacity of about 15 GB. The HD DVD standard uses a light source emitting light with a wavelength of about 405 nm and an objective lens having an NA of 0.65. The thickness of the HD DVD is about 0.6 mm.
An apparatus is needed that can use two optical information storage media, such as a BD and a HD DVD, in one system. In one attempt to address this need, two objective lenses suitable for the two optical information storage media are used to allow the two optical information storage media to be compatible with each other. In this case, however, two objective lenses and other optical parts related to the two objective lenses must be used. Thus, the number of optical parts and the production costs are increased. In addition, it is difficult to fit an optical axis between the two objective lenses.
In order to solve these problems, a method of using only one objective lens and reducing spherical aberration using a hologram optical device may be considered. Japanese Laid-open Patent No. hei 08-062493 discloses a method of allowing a light source for a DVD to be compatible with a CD-family optical disc by using a hologram lens. In this method, a 0th-order diffraction beam is transmitted directly, so as to be focused at a focal point. A +1th-order diffraction beam is transmitted divergently, so as to be focused at a focal point having a different focal distance from the previous focal point.
In the above disclosure, the hologram lens diffracts optical beams, which are incident in the form of parallel optical beams, into 0th-order and +1th-order diffraction beams. The 0th-order diffraction beam is incident as a divergence-free (convergence-free) beam onto an objective lens and is used to record and/or reproduce information on and/or from a relatively thin optical disc. The +1th-order diffraction beam is diverged and used to record and/or reproduce information on and/or from a relatively thick optical disc. An optical spot formed by the 0th-order diffraction beam is used for recording and/or reproduction with respect to a DVD. An optical spot formed by the +1th-order diffraction beam is used for recording and/or reproduction with respect to a CD. The two optical spots are formed on the same optical axis.
As described above, a 0th-order diffraction beam is used for direct transmission and a +1th-order diffraction beam is used for divergent transmission so as to perform recording and/or reproduction with respect to a DVD and a CD using a light source for the DVD. However, although a hologram optical device diffracts incident beams as 0th-order and +1st-order diffraction beams, the amount of the diffraction beams having different orders is not zero. The hologram optical device substantially diffracts the incident beam in smaller amounts of different order beams.
Accordingly, a 0th-order diffraction beam reflected from an optical disc and then incident onto a hologram lens is diffracted by the hologram lens as a 0th-order diffraction beam, a +1st-order diffraction beam, a −1st-order diffraction beam, or the like. The 0th-order diffraction beam is used to detect a signal reproduced from a DVD. The 0th-order/0th-order diffraction beam is used as a signal beam to perform reproduction with respect to the DVD.
Similarly, a +1th-order diffraction beam reflected from the optical disc and then incident onto the hologram lens is diffracted by the hologram lens as a 0th-order diffraction beam, a +1th-order diffraction beam, a −1th-order diffraction beam, or the like. The +1th-order diffraction beam is used to detect a signal reproduced from a CD. The +1st-order/1st-order diffraction beam is used as a signal beam to perform reproduction with respect to the CD.
A signal beam to perform reproduction with respect to a DVD uses 0th-order diffraction beam as an incident beam and a 0th-order diffraction beam generated by being reflected from an optical disc and then incident onto a hologram lens. A signal beam to perform reproduction with respect to a CD uses a +1st-order diffraction beam as an incident beam and a +1st-order diffraction beam generated by being reflected from an optical disc and then incident onto a hologram lens.
If these diffraction beams are used to perform reproduction with respect to a BD and a HD DVD, a 0th-order/0th-order diffraction beam may be used as a signal beam for reproduction with respect to the BD, and the 1st-order/1st-order diffraction beam may be used as a signal beam for reproduction with respect to the HD DVD. However, when an incident nth-order diffraction beam/return beam of nth-order diffraction beam is used as a signal beam by a hologram optical device, an incident beam of n−1th-order diffraction beam/return beam of n+1th-order diffraction beam and an incident beam of n+1th-order diffraction beam/return beam of n−1th-order diffraction beam are also incident onto a photodetector (PD) and thus operate as noise affecting the signal beam. The incident beam is a beam diffracted by the hologram optical device and then radiated onto an optical disc. The reflected beam is a beam reflected from the optical disc, incident onto and diffracted by the hologram optical device, and further proceeding toward the PD.
FIGS. 1A and 1B show optical paths of beams of different diffraction orders diffracted by a hologram optical element (HOE). FIG. 1A shows optical paths of diffracted beams of different diffraction orders in the case of a BD. FIG. 1B shows optical paths of diffracted beams in the case of an HD DVD. In FIGS. 1A and 1B, OL denotes an objective lens.
As shown in FIG. 1A, a parallel beam collimated by a light source is incident onto the HOE. A 0th-order diffraction beam incident from the HOE is reflected from a BD and then is incident onto the HOE through the 0th-order diffraction beam path. The 0th-order diffraction beam advances as a parallel beam after passing through the HOE. A 1st-order diffraction beam incident from the HOE is reflected from the BD and then is incident onto the HOE through a −1st-order diffraction beam path. The −1st-order diffraction beam is incident onto the HOE and then advances as a parallel beam after passing through the HOE. A −1st-order diffraction beam incident from the HOE is reflected from the BD and then is incident onto the HOE through a 1st-order diffraction beam path. This beam advances as a parallel beam after passing through the HOE.
Accordingly, when the BD is used, the 0th-order/0th-order diffraction beam, the −1st-order/1st-order diffraction beam, and the 1st-order/−1st-order diffraction beam advance through the same optical path after passing through the HOE. Thus, when the 0th-order/0th-order diffraction beam is used as a signal beam for reproduction with respect to the BD, the −1st-order/1st-order diffraction beam and the 1st-order/−1st-order diffraction beam operate as noise and interfere with the reproduction of data stored on the BD.
Similarly, as shown in FIG. 1B, when a 1st-order/1st-order diffraction beam is used as a signal beam for reproduction with respect to the HD DVD, a 0th-order/2nd-order diffraction beam and a 2nd-order/0th-order diffraction beam operate as noise.
The noise is created because a size of a spot formed on a PD by the −1st-order/1st-order diffraction beam and the 1st-order/−1st-order diffraction beam with respect to the BD and the 0th-order/2nd-order diffraction beam and the 2nd-order/0th-order diffraction beam with respect to the HD DVD is a similar level according to a size of a spot formed by signal beam. The noise is a main factor in the deterioration of a reproduced signal. The efficiency of diffraction beams generating noise needs to be reduced to reduce noise and to increase the qualities of signals reproduced from the BD and the HD DVD.
When reproduction is performed with respect to a BD having a rewritable dual layer structure, i.e., a BD DL RE (hereinafter referred to as a BD DL), there is flowed noise due to a +1st-order/−2nd-order diffraction beam, a −2nd-order/+1st-order diffraction beam or a −2nd-order/0th-order diffraction beam, and a 0th-order/−2nd-order diffraction beam reflected from a surface of the BD. When reproduction is performed with respect to a BD DL, there may be flowed noise due to diffraction beams of different diffraction orders and noise due to beams reflected from the surface of the BD DL.
FIGS. 2A and 2B show optical paths of beams of different diffraction orders diffracted by a HOE when reproduction is performed with respect to a BD DL. FIG. 2A shows optical paths of diffraction beams when reproduction is performed with respect to a layer L1 of a BD DL. FIG. 2B shows optical paths of diffraction beams when reproduction is performed with respect to a layer L0 of the BD DL. In FIGS. 2A and 2B, a layer L1 is positioned at a distance of 75 μm from a surface of the BD DL, and the layer L0 is positioned at a distance of 25 μm from the layer L1.
As shown in FIG. 2A, when an optical spot is formed by a 0th-order diffraction beam at a distance of about 150 μm from an optical spot formed by a +1st-order diffraction beam in the BD DL, noise due to a +1st-order/−2nd-order diffraction beam and a −2nd-order/+1st-order diffraction beam reflected from a surface of the BD DL is flowed during reproduction with respect to the layer L1.
A +1st-order diffraction beam incident from the HOE is reflected from a surface of the BD DL and then is incident onto the HOE through a −2nd-order diffraction beam path. Thus, a −2nd-order diffraction beam is incident onto the HOE and then advances as a parallel beam. A −2nd-order diffraction beam incident from the HOE is reflected from the surface of the BD DL and then is incident onto the HOE through a +1st-order diffraction beam path. Thus, a +1st-order diffraction beam is incident onto the HOE and then advances as a parallel beam.
Accordingly, the +1st-order/−2nd-order diffraction beam and the −2nd-order/+1st-order diffraction beam advance along the same optical path as the 0th-order/0th-order diffraction beam after passing through the HOE. Thus, the +1st-order/−2nd-order diffraction beam and the −2nd-order/+1st-order diffraction beam reflected from the surface of the BD DL is flowed as noise when reproduction is performed with respect to the layer L1.
As shown in FIG. 2B, when an optical spot is formed by a 0th-order diffraction beam at a distance of about 100 μm from an optical spot formed by a 1st-order diffraction beam in the BD DL, noise due to a 0th-order/−2nd-order diffraction beam and a −2nd-order/0th-order diffraction beam reflected from a surface of the BD DL is flowed during reproduction with respect to the layer L0, resulting in the deterioration of the quality of a reproduced signal.
Therefore, the efficiency of diffraction beams generating noise must be reduced in order to improve the qualities of signals reproduced from a BD and a HD DVD. Noise reflected from a surface of a BD DL must also be reduced in order to improve the quality of a signal reproduced from the BD DL when reproduction is performed with respect to the BD DL.